The Snellen acuity chart is the most widely used form of chart used in clinical practice. In a test the subject is asked to read the lines of letters, each line of which is progressively smaller down the chart, to find the smallest that can be read. This chart has inherent design flaws which result in imprecise and poorly repeatable measurements. Among these flaws ones related to "crowding"--the environment of neighbours surrounding a letter--varying legibility of the particular letters employed and the non-uniform scale of letter sizes used (as opposed to the uniform logarithmic scale appropriate to acuity measurement). These flaws and the non-uniform Snellen fraction measurement scale render acuity data difficult to analyse and the test insensitive to changes over time in a subject's acuity.
The form of visual acuity chart preferred for research is the ETDRS logMAR chart. The ETDRS chart is named after the "Early Treatment Diabetic Retinopathy Study" for which it was originally produced. This chart has fourteen lines of five letters each. In this chart the logarithm to base 10 of the size of the letters in each line reduces by 0.1 (which is a reduction in size by a factor of approximately 1.259). This chart addresses the problems of the Snellen chart stated above, the ETDRS chart providing precise repeatable measurements and readily analysable decimalised acuity data. This ETDRS chart is, however, considered widely too cumbersome for routine clinical use and has not been adopted for such use.